In certain large-scale printing systems, printheads are used to eject ink onto a substrate to create the desired image, for example, on substrates such as museum displays, billboards, sails, bus boards, and banners. In some of these printing systems, the printheads receive ink from an ink supply or reservoir and use a so-called drop on demand ink jet process. With this type of process, ink is ejected from one or more nozzles of the printheads when a piezoelectric crystal in the printhead is actuated. The piezoelectric crystal generates a pulse in the ink so that the ink expels through the nozzle as a droplet. To create the image, a carriage which holds one or more printheads scans or traverses across the substrate while the printheads deposit ink as the substrate moves.
Regardless of the particular type of printing system in which the ink jet printheads are used; the printheads commonly have a series of manifolds and channels that transport the ink to the nozzles. As such, a particular concern for these printheads when the printing system is not in operation is that the ink in the manifolds and channels can harden. That is, water-based inks, as well as solvent-based inks can dry out, or the ink can polymerize if the ink happens to be UV curable. Thus, when the printheads are not in use, it is desirable to somehow seal the printheads to prevent exposing the ink in the printheads to air and/or to UV radiation.
Some have proposed sealing the nozzles of the printheads with a plastic sheet, while others have proposed placing a wet pad soaked with a flushing solvent, such as a solution made of water and glycol ether, against the nozzles when the printheads are not being used. These printhead capping processes are somewhat effective for short periods of time, say, for example, up to one day. However, the performance of these processes has been less than desirable for longer periods of time, for example, three to four days, since it is difficult to keep the printheads completely sealed for these longer periods.
Others have proposed introducing a model solution, such as polypropylene glycol, into the printheads to seal them when the printing system is not in operation. Typically, such solutions are used for shipping or storing the printheads for long periods of time, for example, weeks or months. However, it takes hours and sometimes days to flush the model solution out of the printheads before using them in a printing operation, which may be impractical if the printing system is only inoperative for a few days.